This invention was made during the course of, or under, a contract with the Energy Research and Development Administration.
This invention relates generally to non-contact type proximity detection devices and more specifically to an improved eddy current type proximity detection system in which a printed circuit coil is employed as the sensing medium in the system.
In the art of non-contact eddy current type proximity detection devices, it has generally been the practice of winding probe coils in the form of a doughnut or cylindrical coil with a rectangular cross section. These early coils were primarily utilized for non-conducting displacement or distance measurements of metallic targets at relatively small coil-to-target separations. However, for improved sensitivity and dynamic range in absolute separation measurements, significantly larger coils are required. The fabrication of these larger coils utilizing standard wire techniques presents mechanical, orientation and assembly problems which precluded repeatable probe systems where tight reproducibility requirements are extant. The need for enhanced performance relative to sensitivity, stability and most particularly extended range, mandates the use of larger diameter coils with flatter profiles. The "pancake" or flat, spiral-wound coil reduces internal losses and maximizes the characteristics deemed necessary for improved performance. Orienting wire into a spiral pancake configuration of fixed geometrical relationship is difficult to implement; however, "spiral-wound" flat or pancake type coils are easily, economically and repeatably fabricated using printed circuit (PC) techniques. In addition, the PC technique allows orientation of the conductor maximum width in a plane which maximizes the inductive field flux density.
The electronic circuitry utilized with the proximity device herein described has been designed for maximum stability under a wide range of environmental conditions, as well as optimum sensitivity to changes in electrical performance of the probe coil as influenced primarily by changes in inductive "eddy current" levels.